This invention relates to a screen fabric for use in preparing a stencil which is used for screen printing, textile printing or fabrication of printed circuits.
The stencil for screen printing is, for instance, prepared in accordance with the following processes.
(1) Fabrication of Fiber PA1 (2) Wrapping PA1 (3) Weaving PA1 (4) Heat Set PA1 (5) Stretching PA1 (6) Forming of Photosensitive Layer by Coating of Photosensitized Emulsion or Adhering of Photosensitive Film PA1 (7) Pattern Exposure PA1 (8) Washout PA1 (9) Drying
The fabric is generally prepared to be 100 to 500 meshes when a polyester monofilament is utilized as a material fiber.
In preparing a screen fabric, however, there is a problem to be overcome, which is the peeling off of the scum caused by repeatedly receiving the frictional force during the fabrication process, expecially weaving process thereof. Further, in preparing a stencil, there is also a problem to be overcome, which is the occurance of the so-called "halation", that is the phenomena that the fabric is photosensitized over the predetermined area by the reflection light, which is caused during the pattern exposure process therefor.
When the amount of the peeling off of the scum is excessive, a weaving reed becomes dirty and the fabric is blocked with scum and fibril, which resulting in such troubles as fiber breaking and/or getting dirty of grey goods, so that it becomes difficult to obtain the desired length fabric and the working efficiency becomes remarkably low.
Further, when the halation occurs, not only the quality of the stencil becomes remarkably low but also the precise printing of the printed circuit board becomes difficult.
In order to prevent the peeling off of the scum, the monofilament shall have low content of delustering agents such as titanium dioxide. On the contrary, in order to prevent the occurrence of the halation, it is effective to have high content of the delustering agents in the material fabric. In this connection, in the prior art fabric, the monofilament in semi-dull polymer state having the content of 0.3 to 0.6 weight percent titanium dioxide is generally utilized. However, with the above prior monofilament, it has been difficult to desirably prevent the occurrence of the halation.
Under this circumstance, as it is said that the occurrence of the halation can be prevented when the light reflection ratio is less than 20 percent, it has been proposed to have a fiber dyed in yellow or the like color (Prior Art A). With this prior art A, as indicated by A in FIG. 1, the reflection ratio thereof is less than 20 percent in every light wave length area. The tested sample of the prior art A was prepared with the monofilament including 0.5 weight percent of the titanium dioxide and dyed with the yellow or the like pigment.
Meanwhile, the light of wave length 340-440 nm (hereinafter called exposure light wave length area) is generally utilized to photo-sensitize the screen fabric.
With the above prior art A, however, since such high content of the titanium dioxide as 0.3 to 0.6 weight percent is included in the material fiber, the light transparency level becomes low and the patterns can not be so exposed as the rear side of the fabric is sensitized, and there is a possibility that the surface is worn off from the stencil during printing operations. Accordingly, it becomes necessary to twice expose the pattern on the fabric from the front and rear sides thereof. Further, as the titanium dioxide included in the fibers is partly protruded out of the surface thereof, there remains the defects caused by the peeling off of the scum, abrasion of weaving loom parts, inappropriate ink releasing and so on.
In order to dissolve the above defects of the prior art A, a fiber which does not include the titanium dioxide at all has then been proposed (Prior Art B). This fiber comprises Polyethylene Terephthalates base-polymer contained ultraviolet absorbers therein so as to prevent the occurrence of halation by the contained ultraviolet absorbers. With this prior art B, however, as also indicated in FIG. 1, the light reflection ratio thereof exceeds 20 percent in the visible ray area. As the exposure light wave length area of 340-440 nm includes not only the ultraviolet ray but also the visible ray. Accordingly, it is difficult to prevent the occurrence of the halation in its visible ray area. Further, as the large amount of ultraviolet absorbers is added so as to try to prevent the occurrence of the halation only by the ultraviolet absorbers, it results in increasing of costs, falling down of the quality of the fabric and decreasing of the printing life.